Are You Pulling My Leg?

Content Warning: Self-Mutilation

Betty sits at a table with her hands placed on top of it. As far as an observer can tell, there’s nothing physically wrong with her hands; she has one right and one left, both connected to her arms and body. However, she says that her left hand is not there—it has disappeared. In fact, she can see the table that is underneath where her arm should be [1]. How can this be if her left hand is resting on the table, visible to everyone else? What Betty is experiencing is called asomatognosia, a condition in which a person loses body ownership of a limb. The sense of ownership of one’s body is not a guarantee. In fact, ownership of a limb can be lost, altered, or gained.

The woman, wearing a purple coat, walks her dog in Central Park. A runner runs behind her. The woman sees in a bubble on her head that her left limbs disappear into specks and the runner walks her dog for her.

Fundamentally, body ownership is an awareness of your own body, in which you feel like your body belongs to yourself [2]. It’s your perception of and connection to your own body, where the external and internal sensations you experience feel personal to you [3]. Body ownership is continuous and constant—you experience it during voluntary movement, passive action, and even when stationary [4]. However, body ownership is not absolute; it can easily be altered or manipulated, even if a person is able-bodied and neurotypical [3]. For example, in the Rubber Hand Illusion, a participant can be momentarily tricked into believing that a fake rubber hand is their own. This experiment involves placing a participant’s real hand and a fake rubber hand side by side on a table. The real hand is hidden behind a screen, while the fake rubber hand is visible in front of them. Then, an experimenter strokes both the hidden real hand and visible fake hand at the same time, causing the participant to connect what they feel in their real hand to what they see in the fake hand. Subsequently, the participant briefly adopts the fake hand as their own [3]. Ultimately, the experiment proves a crucial aspect of body ownership: it can be modified.

Where’s my leg?

Stemming from the Greek phrase “without body knowledge,” asomatognosia is a condition in which a patient loses a sense of agency over their limb [5]. For instance, patient ASG, hereafter referred to as Anne, was a highly educated 53-year-old woman who had undergone surgery to remove a brain tumor [6]. A few months after the surgery, while walking her dog, she suddenly felt that the limbs on the left side of her body did not exist. It was as if they were completely gone. Though Anne was still able to see her left hand and leg, she sensorially felt that they were no longer there. Consequently, she was often left wondering how she could continue to walk her dog with her left hand, despite it being absent [6]. Anne’s case was one of sensory asomatognosia. Sensory asomatognosia is when a person doesn’t feel their limb; in contrast, visual asomatognosia is when a person doesn’t see their limb. In the example mentioned earlier, Betty was experiencing visual asomatognosia because she could not see her arm [1]. Research also reports that some patients with asomatognosia could not identify their own hand while it was placed on a table next to a researcher’s hand [5]. Regardless, in all cases of asomatognosia, the patient lacks some perceptual capability surrounding their own limb.

People often develop asomatognosia after suffering from damage to the right hemisphere of the brain [5]. Since the right hemisphere deals with sensations and information coming from the left side of the body, patients usually experience asomatognosia on the left side of their body [7]. This relationship between the right hemisphere of the brain and the left side of the body is important for the conditions regarding body ownership mentioned later in this article. While patients with asomatognosia tend to have damage to various parts of the right hemisphere, including areas such as the medial frontal region, one particularly relevant area that patients typically incur damage to is the temporo-parietal section of the brain [5]. The parietal lobe deals with awareness of the position of the body in space, while the temporal lobe is involved with auditory and some visual processing [7]. Most importantly, however, is that the junction between the temporal lobe and parietal lobe is often associated with the ability to orient oneself and control attention [8]. Thus, damage to these areas may result in a patient losing their ability to localize and focus on their limb.

A person's left limbs and chest (in the hue of purple) overlaps with a person's right limbs and chest (in hue of yellow).

That’s not my leg, it’s hers!